Pulsed laser source with adjustable grating compressor

ABSTRACT

Various embodiments described herein relate to a laser source for producing a pulsed laser beam comprising a plurality of ultrashort optical pulses having a variable repetition rate. In one embodiment, the laser source comprises a fiber oscillator, which outputs optical pulses and a pulse stretcher disposed to receive the optical pulses. The optical pulses have an optical pulse width. The pulse stretcher has dispersion that increases the optical pulse width yielding stretched optical pulses. The laser source further comprises a fiber amplifier disposed to receive the stretched optical pulses. The fiber optical amplifier has gain so as to amplify the stretched optical pulses. The laser source includes an automatically adjustable grating compressor having dispersion that reduces the optical pulse width. The grating compressor automatically adjusts this dispersion for different repetition rates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/314,197, filed Dec. 20, 2005, entitled “PULSED LASER SOURCE WITHADJUSTABLE GRATING COMPRESSOR,” which claims priority under 35 U.S.C.§119(e) to U.S. Provisional Patent Application No. 60/638,072 filed Dec.20, 2004, entitled “OPTICAL WAVEGUIDE WRITING UTILIZING VISIBLEFEMTOSECOND PULSES,” each of which is hereby expressly incorporated byreference herein in its entirety.

BACKGROUND

1. Field of the Invention

The apparatus and methods relate to pulsed lasers and to fabricatingwaveguides with pulsed lasers.

2. Description of the Related Art

As is well known, waveguides comprise optical pathways for propagatinglight within a medium such as, for example, within a slab. The waveguidemay be fabricated by modifying a refractive index of a material in themedium. Examples of waveguides include channel waveguides that may bedisposed at a surface of or buried within a substrate. Integrated opticswith complicated 3-D architectures may be formed with such waveguidesstructures.

Waveguides may be fabricated by shining light into the medium to alterthe physical state of the material and to modify the refractive index inthe material. Waveguide writing may be accomplished by using lasers and,in particular, pulsed lasers. Unfortunately, waveguide writing is acomplex process. The process may not be feasible for many waveguidematerials using commercially available laser systems. Accordingly,additional apparatus and methods are needed that enable waveguidewriting with laser light.

SUMMARY

Various embodiments described herein include a laser source forproducing a pulsed laser beam comprising a plurality of ultrashortoptical pulses having a variable repetition rate. In one embodiment, thelaser source comprises a fiber oscillator, which outputs optical pulsesand a pulse stretcher disposed to receive the optical pulses. Theoptical pulses have an optical pulse width. The pulse stretcher hasdispersion that increases the optical pulse width yielding stretchedoptical pulses. The laser source further comprises a fiber amplifierdisposed to receive the stretched optical pulses. The fiber opticalamplifier has gain so as to amplify the stretched optical pulses. Thelaser source includes an automatically adjustable grating compressorhaving dispersion that reduces the optical pulse width. The dispersionof the grating compressor is adjustable. The grating compressorautomatically adjusts the dispersion for different repetition rates.

Another embodiment of the invention also comprises a laser source thatproduces a pulsed laser beam comprising a plurality of ultrashortoptical pulses having a variable repetition rate. The laser sourcecomprises a fiber oscillator that outputs optical pulses having anoptical pulse width and an automatically adjustable grating compressorhaving dispersion that reduces the optical pulse width. The dispersionis adjustable, and the grating compressor automatically adjusts thedispersion for different repetition rates.

Another embodiment comprises a method of producing a pulsed laser beamcomprising a plurality of ultrashort optical pulses having a variablerepetition rate. The method comprises producing optical pulses having anoptical pulse width, reducing the optical pulse width to providecompressed optical pulses, and varying the repetition rate. In thismethod, the dispersion of a compressor is automatically adjusted for thedifferent repetition rates so as to produce the minimum pulse width.

Another embodiment described herein comprises a method of fabricating awaveguide in a medium. This method comprises producing an ultrafastpulsed laser beam comprising optical pulses having a pulse width betweenabout 300 and 700 femtoseconds in duration and a wavelength in the rangebetween about 490 and 550 nanometers. This method further comprisesdirecting at least a portion of the ultrafast pulsed laser beam into aregion of the medium and removing the ultrafast pulsed laser beam fromthe region of the medium. The ultrafast pulsed laser beam directed intothe region has sufficient intensity to alter the index of refraction ofthe medium in the region after the ultrafast pulse laser beam is removedso that the waveguide is formed in the medium. In one embodiment of thismethod, the ultrafast pulsed laser beam has a laser fluence on theregion of the medium of between about 5 J/cm² and 50 J/cm². In anotherembodiment, the optical pulses have a repetition rate between about 100kHz to 5 MHz.

Another embodiment of this method of fabricating a waveguide furthercomprises producing infrared light and frequency doubling the infraredlight to produce the visible light beam.

In another embodiment of this method of fabricating a waveguide, themedium is selected from the group of materials consisting ofsubstantially transparent crystal, glass, and polymer. In anotherembodiment, the medium comprises fused silica. In yet anotherembodiment, the medium comprises a material having a material ionizationbandgap λ_(g) and the wavelength is between 3.0λ_(g) and 5λ_(g).

In another embodiment, this method further comprises translating themedium to form elongated waveguides.

In yet another embodiment, this method further comprises moving theultrafast pulsed laser beam to form elongated waveguides.

Another embodiment disclosed herein comprises a system for fabricating awaveguide in a medium. This system comprises an infrared fiber laser, afrequency doubler, and a translation system. The infrared fiber laseroutputs an ultrafast pulsed infrared laser beam. The frequency doublerreceives the ultrafast pulsed infrared laser beam and outputs anultrafast pulsed visible laser beam having a wavelength in the rangebetween about 490 and 550 nanometers. The ultrafast pulsed visible laserbeam comprises optical pulses having a pulse width of between about 300and 700 femtoseconds in duration. The ultrafast pulsed visible laserbeam illuminates a spatial region of the medium. The translation systemalters the spatial region to form the waveguide in the medium.

In one embodiment of this system, the infrared fiber laser comprises aYb-doped fiber laser. In yet another embodiment, the infrared fiberlaser outputs a wavelength between 1030 and 1050 nanometer and thefrequency doubler comprises a nonlinear optical element that produceslight with a wavelength between 515 and 525 nanometers through secondharmonic generation.

In another embodiment of this system for fabricating a waveguide, theultrafast pulsed visible laser beam has a laser fluence on the spatialregion of between about 5 J/cm² and 50 J/cm². In yet another embodiment,the optical pulses have a variable repetition rate from about 100 kHz to5 MHz.

Another embodiment of this system for fabricating a waveguide furthercomprises optics disposed to receive the ultrafast visible pulsed laserbeam and illuminate the spatial region therewith. In one embodiment ofthis system, the optics comprises a microscope objective. In anotherembodiment of the system, the optics has a numerical aperture of lessthan about 1.0.

In one embodiment of this system for fabricating a waveguide, thetranslation system comprises a translation stage on which the medium isdisposed. In yet another embodiment, the translation system comprises amovable mirror.

Also disclosed herein is a system for fabricating a waveguide thatcomprises an ultrafast pulsed laser light source that produces anultrafast pulsed visible laser beam. The ultrafast pulsed visible laserbeam comprises optical pulses having a pulse duration between about 300to about 800 femtoseconds and a wavelength between about 490 and 550nanometers. This system further comprises a medium positioned in thebeam. The medium has a physical structure and an index of refractionthat depends on the structure such that the structure is altered by thebeam of visible light to thereby alter the index of refraction.

In an embodiment of this system for fabricating a waveguide, the mediumhas a material ionization bandgap λ_(g) and the wavelength is between3.0λ_(g) and 5λ_(g). In another embodiment, the medium is selected fromthe group of materials consisting of transparent crystal, glass, andpolymer, while in yet another embodiment, the medium comprises fusedsilica.

Also disclosed herein is a method of fabricating a waveguide in a mediumcomprises producing a visible light beam and directing the visible lightbeam into a region of the medium to alter a physical state of the mediumin the region and change the refractive index in the region therebyforming a waveguide.

In one embodiment of this method of fabricating a waveguide, the lightsource has an output wavelength between 3.0λ_(g) and 5λ_(g), where λ_(g)is the material ionization bandgap. In another embodiment, the visiblelight beam comprises green light. In still another embodiment, thevisible light beam has a wavelength in the range between about 490 and550 nanometers for waveguide writing in fused silica.

Still another embodiment disclosed herein comprises a method offabricating a waveguide in a medium. This method comprises producing avisible light beam, directing at least a portion of the visible lightbeam into a region of the medium, and removing the visible light beamfrom the region of the medium. The visible light beam directed into theregion has sufficient intensity to alter the index of refraction of themedium in the region after the visible light beam is removed to form thewaveguide in the medium.

In one embodiment of this method of fabricating a waveguide, the lightsource has an output wavelength between 3.0λ_(g) and 5λ_(g), where λ_(g)is the material ionization bandgap. In another embodiment of thismethod, the visible light beam comprises green light. In yet anotherembodiment, the visible light beam has a wavelength in the range betweenabout 490 and 550 nanometers for waveguide writing in fused silica.

Another embodiment of this method of fabricating a waveguide furthercomprises producing infrared light and frequency doubling the infraredlight to produce the visible light beam. In another embodiment of thismethod, the frequency doubling comprises second harmonic generation. Inanother embodiment, the infrared light comprises laser light of about1040 nanometers and the visible light beam comprises laser light ofabout 520 nanometers. In yet another embodiment, the method furthercomprises pulsing the infrared laser at a variable repetition rate fromabout 100 kHz to 5 MHz. In still another embodiment, the infrared laserincludes a compressor grating and a translator that is automaticallyrepositioned to provide optimal pulse compression with changes in therepetition rate.

An embodiment of this method of fabricating a waveguide furthercomprises pulsing the visible light beam to produce pulses between about300 and 700 femtoseconds in duration. In another embodiment of thismethod, the medium is selected from the group of materials consisting ofsubstantially transparent crystal, glass, and polymer. In yet anotherembodiment of this method, the medium comprises fused silica.

An additional embodiment of this method of fabricating a waveguidefurther comprises translating the medium to form elongated waveguides.Another embodiment of this method further comprises moving the visiblelight beam to form elongated waveguides. In yet another embodiment ofthis method, the index of refraction in the medium is increased afterremoval of the visible light.

Another embodiment disclosed herein comprises a system for fabricating awaveguide. This system comprises a light source that produces a beam ofvisible light and a medium positioned in the beam. The medium has aphysical structure and an index of refraction that depends on thestructure. The structure is altered by the beam of visible light suchthat the index of refraction is altered.

In one embodiment of this system, the light source has an outputwavelength between 3.0λ_(g) and 5λ_(g), where λ_(g) is the materialionization bandgap. In another embodiment of this system, the lightsource has an output wavelength between about 490 and 550 nanometers forwaveguide writing in fused silica. In yet another embodiment, the lightsource comprises a laser. In still another embodiment, the medium isselected from the group of materials consisting of substantiallytransparent crystal, glass, and polymer. In an embodiment of thissystem, the medium comprises fused silica.

Another embodiment disclosed herein also comprises a system forfabricating a waveguide in a medium. This system comprises a visiblelaser light source that outputs a visible light and illuminates aspatial region of the medium with the visible light and a translationsystem for translating the spatial region to form the waveguide in themedium.

In one embodiment of this system for fabricating a waveguide, thevisible laser light source comprises an infrared laser and a frequencydoubler. In another embodiment, the infrared laser comprises a Yb-dopedfiber laser. In yet another embodiment, the infrared laser comprises a1045 nanometer wavelength laser and the frequency doubler comprises anonlinear optical element that produces 522.5 nanometer wavelength lightthrough second harmonic generation. In still another embodiment, thevisible laser light source has an output wavelength between about 490and 550 nanometers. For example, the infrared fiber laser may output awavelength between 1030 and 1050 nanometer and the frequency doublercomprises a nonlinear optical element may produce light with awavelength between 515 and 525 nanometers through second harmonicgeneration. In a further embodiment, the visible laser light source hasa variable repetition rate from about 100 kHz to 5 MHz.

In another embodiment of this system for fabricating a waveguide, theinfrared laser includes a compressor grating and a mechanism by whichthe dispersion of compressor grating is automatically adjusted toprovide optimal pulse compression with changes in the repetition rate.

Another embodiment of this system further comprises optics disposed toreceive the visible light output from the visible laser light source andilluminate the spatial region with the visible light. In one embodiment,the optics comprises a microscope objective. In another embodiment, theoptics has a numerical aperture of less than about 1.0.

In another embodiment of this system of fabricating a waveguide, thetranslation system comprises a translation stage on which the medium isdisposed. In another embodiment of this system, the translation systemcomprises a movable mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a system for writing a waveguide in amedium, which comprises a visible laser light source, focusing optics,and a translation system that supports the medium.

FIG. 2 shows a schematic diagram of an embodiment of a visible laserlight source that comprises an oscillator, a pulse stretcher, an opticalamplifier, and a grating compressor.

FIG. 3A shows a schematic diagram of one embodiment of the gratingcompressor that comprises first and second gratings and a mirror.

FIG. 3B shows a schematic diagram of one embodiment of the gratingcompressor that comprises one grating and two retroreflectors.

FIG. 4 shows an embodiment of the system for writing a waveguide inwhich the translation system comprises a rotating or tilting mirror.

FIG. 5 shows an embodiment of the system for writing a waveguide thatuses a mask to form a pattern in or on the medium.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

A system 10 capable of writing a waveguide in a medium comprising fusedsilica is illustrated in FIG. 1. This system 10 comprises a visiblelaser light source 12 that outputs visible light. The visible laserlight source 12 has an output wavelength in the green region of thevisible optical spectrum, e.g., between about 490 and 550 nanometers.This wavelength range may also be between about 450 and 700 nanometersin some embodiments.

The visible laser light source 12 comprises a Yb-doped fiber laser 14that outputs light having a wavelength of approximately 1045 nanometers.An exemplary Yb-doped, amplified fiber laser 14 comprises the FCPAμJewel available from IMRA America, Ann Arbor Mich. This fiber laser hasa pulse repetition rate between about 100 kHz and 5 MHz and is capableof outputting ultrashort pulses having pulse durations between about 300fs and 700 fs. The high repetition rate allows the fabrication oflow-loss waveguides at relatively high translation speeds (˜1 mm/s).Values outside these ranges may also be possible in other embodiments.

The visible laser light source 12 further comprises a frequency doubler16 that receives the optical pulses from the Yb-doped fiber laser. Onepreferred embodiment of the frequency doubler 16 utilizes non-criticallyphase matched lithium triborate (LBO) as the nonlinear media as this canmaximize conversion efficiency and output beam quality. The frequencydoubler may also comprise nonlinear media such as beta-barium borate(BBO), potassium titanyl phosphate (KTP), bismuth triborate BiB₃O₆,potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate(KD*P), potassium niobate (KNbO₃), lithium niobate (LiNbO₃) and mayinclude appropriate optics to focusing the incident beam into thenonlinear medium, to increase conversion efficiency, and collimate thesecond harmonic output beam. In some embodiments, the frequency doublerproduces a frequency doubled output at a wavelength of about 522 nmthrough second harmonic generation. This output from the frequencydoubler 16 and from the visible laser light source 12 is shown as a beam18 in FIG. 1.

Other types of light sources and specifically other types of visiblelaser light sources 12 may be employed. Other types of lasers may beemployed. For example, other types of fiber and non-fiber pulsed lasersmay be employed. Frequency doubling and second harmonic generation mayor may not be employed in different embodiments.

The system 12 further includes optics 20 disposed to receive visiblelight output from the visible laser light source 12. The optics 20 mayinclude, for example, a microscope objective that focuses the beam 18into a target region 22. (Note that the drawing in FIG. 1 is schematicand does not show the convergence of the beam although optics thatfocuses the beam may be employed.)

The optics 20 may have a numerical aperture (NA) less than about 1.0 andbetween about 1.0 and 0.4 in some embodiments. The low NA focusingobjective facilitates the fabrication of three-dimensional waveguidepatterns due to the longer depth of focus relative, e.g., to oil- andwater-immersed objectives with NA>1.0. The visible wavelength near about520 nm is also more compatible with standard high magnificationobjectives used in visible microscopy than near infrared (NIR)wavelengths. As such, the insertion loss and beam aberration introducedby the objective is significantly reduced. Other types of optics 20 maybe employed and the optics may be excluded in certain embodiments.

The system 10 directs the laser beam onto a medium 24 and, inparticular, into the target region 22 in or on the medium to form thewaveguide. This medium 24 may comprise fused silica in some embodiments.The medium 24 may also comprise glass or polymer as well as crystal.Examples of material that may be employed include fluorine-doped silicaglass and high bandgap crystalline materials such as quartz, sapphire,calcium fluoride, magnesium fluoride, barium fluoride, and beta bariumborate. Other material may be used.

The system 10 further comprises a translation system 26 for moving thetarget region 22. The medium 24 may, for example, be mounted on atranslation stage 28 that is translated or otherwise moved with respectto the laser beam 18. In other embodiments, the laser beam 18 may betranslated, for example, using a mirror that can be rotated or tilted.The laser beam 18 may be translated or moved by moving other opticalelements, for example, by shifting the microscope lens 20. Otherconfigurations and arrangements for moving the beam 18 with respect tothe medium 24 or otherwise moving the target region 22 may be employed.

The visible light incident on the medium 24 alters the index ofrefraction of the medium 24. In various preferred embodiments,illumination of the target region 22 with the visible laser light altersthe physical state of the medium 24 and increases the index ofrefraction in the target region 22. The index of refraction is changedor increased in comparison with surrounding portions of the medium 24not illuminated with the visible laser light. Because the physical stateof the medium 24 is altered, the laser beam 18 can be removed and thechange or increase in refractive index remains. A number of mechanismsmay be possible for causing the index to be changed with illuminationusing the visible light.

Systems 10 such as described above offer many advantageous technicalfeatures. Use of frequency doubled 1045-nanometer radiation, forexample, provides numerous benefits. For instance, while waveguideswritten in fused silica with the fundamental wavelength of a Yb-dopedfiber laser (1045 nm) result in high optical losses, waveguides writtenunder similar conditions with the second harmonic of the same laser (522nm) produce very low optical losses.

The shorter wavelength also allows for tighter focusing due to thereduction in the diffraction limited spot size. Achieving high focalintensity/fluence with relatively low incident pulse energy is thereforepossible.

Shorter wavelength should also enhance the multiphoton absorptionprocess relative the longer wavelengths. Shorter wavelengths producelower-order multi-photon ionization, leading to a wider separationbetween non-linear absorption states. This increases the separationbetween the thresholds for refractive index modification or damageallowing for a larger processing window. Interestingly, however, inother experiments ultrashort pulse, 400-nm laser irradiation is notsuccessful in writing waveguides.

Also, since substantially many objectives have been designed forbiological microscopy, the performance of these microscope objectives(such as optical transmission and aberration correction) is improved oroptimized for visible wavelengths. Accordingly, by using the secondharmonic of the Yb-based laser, the wavelength allows for simpleintegration into existing microscopic systems. Waveguide writing cantherefore be integrated in parallel together with a rudimentaryinspection system.

Waveguide writing is a complex and challenging process. As referred toabove, techniques that may be well-suited for one class of materials maybe inappropriate for another class. Accordingly, identifying this regimewherein waveguide writing is possible, appears to provide benefits suchas, for example, lower optical losses, use of smaller spot sizes andhigher focal intensity/fluences, and improved integration into existingmicroscope systems, etc., that might not otherwise available.

In one preferred embodiment, a laser source 200 such as schematicallyshown in FIG. 2 comprises, for example, a modified FCPA μJewel from IMRAAmerica. Additional details regarding a variety of laser sources 200 aredisclosed in U.S. patent application Ser. No. 10/992,762 entitled“All-Fiber Chirped Pulse Amplification Systems” (IM-114), filed Nov. 22,2004, and U.S. Pat. No. 6,885,683 entitled “Modular, High Energy,Widely-tunable Ultrafast Fiber Source,” issued Apr. 26, 2005, both ofwhich are incorporated herein by reference in their entirety. Generally,such a laser source 200 comprises an oscillator 210, a pulse stretcher220, an optical amplifier 230, and a grating compressor 240.

The oscillator 210 may comprise a pair of reflective optical elementsthat form an optical resonator. The oscillator 210 may further include again medium disposed in the resonator. This gain medium may be such thatoptical pulses are generated by the oscillator 210. The gain medium maybe optically pumped by a pump source (not shown). In one embodiment, thegain medium comprises doped fiber such as Yb-doped fiber. The reflectiveoptical elements may comprise one or more mirrors or fiber Bragggratings in some embodiments. The reflective optical elements may bedisposed at the ends of the doped fiber. Other types of gain mediums andreflectors as well as other types of configurations may also be used.The oscillator 210 outputs optical pulses having a pulse duration orwidth (full width half maximum, FWHM), τ, and a repetition rate, Γ.

The pulse stretcher 220 may comprise an optical fiber having dispersion.The pulse stretcher 220 is optically coupled to the oscillator 210 anddisposed to receive the optical pulses output by the oscillator. Incertain embodiments, the oscillator 210 and the pulse stretcher 220 areoptical fibers butt coupled or spliced together. Other arrangements andother types of pulse stretchers 220 may also be used. The output of thepulse stretcher is a chirped pulse. The pulse stretcher 220 increasesthe pulse width, τ, stretching the pulse, and also reduces the amplitudeof the pulse.

The pulse stretcher 220 is optically coupled to the amplifier 230 suchthat the amplifier receives the stretched optical pulse. The amplifier230 comprises a gain medium that amplifies the pulse. The amplifier 230may comprise a doped fiber such as a Yb-doped fiber is some embodiments.The amplifier 230 may be optically pumped. A same or different opticalpump source may be used to pump the oscillator 210 and the amplifier230. The amplifier 230 may be non-linear and may introduce self-phasemodulation. Accordingly, different amplitude optical pulses mayexperience different amounts of phase delay. Other types of amplifiersand other configurations may be used.

The grating compressor 240 is disposed to receive the amplified opticalpulse from the optical amplifier 230. Different types of gratingcompressors 240 are well known in the art. The grating compressor 240comprises one or more gratings that introduce dispersion and isconfigured to provide different optical paths for different wavelengths.The grating compressor 240, which receives a chirped pulse, may beconfigured to provide for phase delay of longer wavelengths (e.g.,temporally in the front of the optical pulse) that is different than thephase delay of the shorter wavelengths (e.g., temporally in the rear ofthe optical pulse). This phase delay may be such that in the pulseoutput from the compressor, the longer and short wavelengths overlaptemporally and the pulse width is reduced. The optical pulse is therebycompressed.

In one preferred embodiment, the laser source 200 comprises a Yb-doped,amplified fiber laser (e.g., a modified FCPA μJewel, available from IMRAAmerica). Such a laser offers several primary advantages over commercialsolid-state laser systems. For example, this laser source provides avariable repetition rate that spans a “unique range” from about 100 kHzto 5 MHz. Additionally, higher pulse energy than oscillator-only systemsallows greater flexibility in focal geometry. Higher repetition ratethan solid-state regeneratively amplified systems allow greaterfabrication speed. The variable repetition rate also facilitates theoptimization of the index modification conditions for different mediums,e.g., different glasses etc.

In one embodiment of the laser source 200, the pulse is stretched with alength of conventional step-index single-mode fiber and compressed withthe bulk grating compressor 240. The large mismatch in third-orderdispersion between the stretcher 220 and compressor 240 is compensatedvia self-phase modulation in the power amplifier 230 through the use ofcubicon pulses. The cubicon pulses have a cubical spectral and temporalshape. Under the influence of self-phase modulation in the poweramplifier 230, the triangular pulse shape increases the nonlinear phasedelay for the blue spectral components of the pulses while inducing amuch smaller nonlinear phase delay for the red spectral components. Thedegree of this self-phase modulation depends on the intensity of thelaser pulse within the power amplifier 230. Moreover, variation in therepetition rate will cause a change in the intensity and, thus, alsoalter the phase delay and dispersion.

For constant average power, P_(avg), resulting in large part fromconstant pumping, P_(avg)=E_(pulse)×Γ, where E_(pulse) is the pulseenergy (J) and Γ is the repetition rate (Hz). Thus for constant averagepower, increasing the repetition rate causes the pulse energy todecrease. Conversely, decreasing the repetition rate causes the pulseenergy to increase. Given that the pulse energy changes with repetitionrate, e.g., from 3 μJ at 100 kHz to 150 nJ at 5 MHz, the degree ofself-phase modulation also changes. The change in self-modulation in theamplifier 230 causes the pulse width to change. To correct for thischange in pulse width caused by the variation in repetition rate, thedispersion of the grating compressor 240 can be adjusted.

FIG. 3A schematically illustrates one embodiment of the gratingcompressor 240 that automatically adjusts the dispersion of the gratingcompressor with change in repetition rate. The grating compressor 240includes first and second gratings 242, 244, and a mirror 246. Asillustrated, an optical path extends between the first and secondgratings 242, 244 and the mirror 246. Accordingly, a beam of light 248received through an input to the grating compressor 240 is incident onthe first grating 242 and diffracted therefrom. The beam 248 issubsequently directed to the second grating 244 and is diffractedtherefrom toward the mirror 246. The beam 248 is reflected from themirror 246 and returns back to the second grating 244 and is diffractedtherefrom to the first grating 242. This beam 248 is then diffractedfrom the first grating 242 back through the input.

FIG. 3A shows the second grating 244 disposed on a translation stage 250configured to translate the second grating in a direction represented byarrow 252. The translation stage 250 is in communication with acontroller 254 that controls the movement of the translation stage. Thecontroller 254 is also in communication with a storage device 256. Thecontroller 254 may comprise a processor, microprocessor, CPU, computer,workstation, personal digital assistant, pocket PC, or other hardwaredevices. The controller 254 may implement a collection of instructionsor processing steps stored in hardware, software, or firmware. Thecollection of instructions or processing steps may be stored in thecontroller 254 or in some other device or medium.

The collection of instructions or processing steps may include computerprogram code elements and/or electronic logic circuits. Variousembodiments of the controller 254 include a machine component thatrenders the logic elements in a form that instructs a digital processingapparatus (e.g., a computer, controller, processor, workstation, laptop,palm top, personal digital assistant, cell phone, kiosk, or the like,etc.) to perform a sequence of function steps. The logic may be embodiedby a computer program that is executed by the processor as a series ofcomputer- or control element-executable instructions. These instructionsor data usable to generate these instructions may reside, for example,in RAM or on a hard drive or optical drive, or on a disc or theinstructions may be stored on magnetic tape, electronic read-onlymemory, or other appropriate data storage device or computer accessiblemedium that may or may not be dynamically changed or updated.Accordingly, these methods and processes may be included, for example,on magnetic discs, optical discs such as compact discs, optical discdrives or other storage device or medium both those well known in theart as well as those yet to be devised. The storage mediums may containthe processing steps which are implemented using hardware. Theseinstructions may be in a format on the storage medium, for example, datacompressed, that is subsequently altered.

Additionally, some or all of the processing can be performed all on thesame device, on one or more other devices that communicates with thedevice, or various other combinations. The processor may also beincorporated in a network and portions of the process may be performedby separate devices in the network. Display of information, e.g., a userinterface, can be included on the device, or the information can becommunicated to the device, and/or communicated with a separate device.

The storage device 256 may comprise one or more local or remote devicessuch as, for example, disk drives, volatile or nonvolatile memory,optical disks, tapes, or other storage device or medium both those wellknown in the art as well as those yet to be devised. Communicationbetween the storage device 256 and the controller 254 may be via, e.g.,hardwiring or by electro-magnetic transmission and may be, e.g.,electrical, optical, magnetic, or microwave, etc. Similarly,communication between the controller 254 and the translation stage 252may be via, e.g., hardwiring or by electro-magnetic transmission and maybe, e.g., electrical, optical, magnetic, or microwave, etc. A widevariety of configurations and arrangements are possible.

FIG. 3A also shows arrow 258 representing translation of the firstgrating 242. Either or both of these gratings 242, 244 may be translatedusing translators connected to the controller 254 or other controllers.Such translation of the first and/or second gratings 242, 244 changesthe separation therebetween, which increase or decreases the opticalpath length traveled by the light between the gratings. Increasing ordecreasing this optical path length increases or decreases the effectsof the angular dispersion of the gratings on the beam. In certainembodiments, the mirror 246 may also be translated.

In various preferred embodiments, the storage device 256 contains adatabase that includes values representative of repetition rates andvalues representative of position or translation amounts for thetranslation stage 250. When the repetition rate is set or changed, e.g.,by a user, the controller 254 may access the storage device 256 andreceive therefrom values used to automatically adjust the position ofthe translator stage 250 and/or the positions of one or both gratings242, 244.

As described above, in the embodiment of the compressor grating 240shown in FIG. 3A, translation of the grating 244 as indicated by thearrow 252 alters the optical path distance that diffracted lightpropagates between the gratings. Changing this optical path lengthalters the dispersion introduced to the beam 248 by the gratingcompressor 240. Accordingly, translating the second grating 244different amounts using the translator 250 alters the dispersion of thegrating compressor 240 and may be used to compensate for variation indispersion of other portions of the laser source 200. In particular, thecontroller 254 may be configured to automatically induce translation ofthe second grating 244 via the translator 250 by an appropriate amountin response to a change in the repetition rate so as to counter thechange in dispersion in the amplifier 530 that results from the changein the repetition rate.

Different configurations are possible. With reference to FIG. 3A,different combinations of the gratings 242, 244 and the mirror 246 maybe translated to automatically adjust the dispersion of the gratingcompressor 240 by altering the optical path of the beam 248, e.g.,between the gratings.

Additionally, the grating compressor 240 may be designed differently.Either of the gratings 242, 244 and the mirror 246 may be excluded. Inanother embodiment, for example, the grating compressor 240 comprisesthe first and second gratings 242, 244 without the mirror 246. In otherembodiments, more gratings may be used. Additionally, in otherembodiments, the grating compressor 240 comprises the first grating 242and the mirror 246 without the second grating 244. Other designs arealso possible. For example, a prism may be used in place of the mirror.The prism may facilitate output of the pumped laser beam 248 from thegrating compressor 240 and laser source 200. Still other configurations,both well known in the art as well as those yet to be devised may beused.

FIG. 3B illustrates another embodiment of the compressor grating 240that comprises a grating 242 and first and second retroreflectors 272,274. The first retroreflector 272 is disposed on a translation stage250, which is configured to translate the retroreflector 272 in thedirection represented by the arrow 252. The translation stage 250 may beconfigured to operate in a substantially similar manner to thatdescribed with reference to FIG. 3A. The incident light beam 248 isreceived from an input to the grating compressor 240 and travels alongan optical path to the grating 242 and is diffracted therefrom. The beam248 subsequently travels to the first retroreflector 272 and isredirected back toward the grating 242. The beam 248 is diffracted fromthe grating 242 and travels towards the second retroreflector 274. Thebeam 248 reflects from the second retroreflector 274 and reverses itspath through the grating compressor 240 and back through the input.

The retroreflectors 272, 274 may comprise prisms that in addition toreflecting the beam, provide that the reflected beam is laterallydisplaced with respect to the incident beam. Accordingly, an input beamcan thereby be separated from an output beam. In FIG. 3B, the firstretroreflector 272 is oriented to laterally separate the input andoutput beams in the horizontal direction (in the plane of the paper).The second retroreflector 274 shown in FIG. 3B also comprises a prism;this prism is oriented so as to laterally separate the input and outputbeams in the vertical direction (out of the plane of the paper).Accordingly, the first and second retroreflectors 272, 274 may comprisesubstantially identical prisms that are oriented differently (e.g.rotated 90°) with respect to each other. Other types of retroreflectorsand other configurations may also be used.

Translation of the first retroreflector 272 as indicated by the arrow252 alters the optical path distance traveled by the beam 248 betweenreflections from the grating 242 and thus alters the dispersionintroduced to the beam 248 by the compressor grating 240. Accordingly,translating the first retroreflector 272 different amounts using thetranslator 250 alters the dispersion of the grating compressor 240.Moreover, the first retroreflector 272 may be operably coupled to acontroller (not shown but which may be generally similar to thecontroller 254) that may be configured to automatically inducetranslation of the first retroreflector 272 by an appropriate amount inresponse to a change in the repetition rate so as to counter the changein dispersion in the amplifier 530 that results from the change in therepetition rate. Other aspects of the operation of the gratingcompressor 240 shown in FIG. 3B may be generally similar to those of thegrating compressor 240 shown in FIG. 3A. Variations are also possible.

Additionally, instead of using a storage device 256 that containsinformation relating to the repetition rate upon which the controller254 uses to automatically adjust the translation stage 250 in thegrating compressor 240, the controller may use a different arrangementfor determining the appropriate displacement of the translator 250. Forexample, an optical detector (e.g., a photodiode) may be included thatmonitors the repetition rate. The controller 254 may use thisinformation from the optical detector. In other embodiments, the opticaldetector provides a measure of the pulse width and the controller 254uses this information to automatically adjust the dispersion of thegrating compressor 240. Thus, a feedback system that includes theoptical detector and the controller 254 may be included to automaticallyadjust the dispersion of the grating compressor 240. Additional detailsregard using feedback to control the laser system 200 is disclosed inU.S. patent application Ser. No. 10/813,269 entitled “Femtosecond LaserProcessing System with Process Parameters, Controls and Feedback,”(IM-110) filed Mar. 31, 2004, which is incorporated herein by referencein its entirety. Other variations in design are possible.

Although the laser source 200 comprising the automatic gratingcompressor that automatically adjusts for variation in repetition ratemay be used in fabricating waveguides as disclosed herein, the lasersource is not so limited and may be used for other applications as well.

This laser source 200 may, however, be particularly useful forfabricating waveguide. The combination of high repetition rate andrelatively high pulse energy makes possible writing low loss waveguidesat high speeds (˜1 mm/s) without the need for high NA focal objectives.The ability to use relatively low NA focal objectives simplifies theoptical layout and provides long working distance and long depth offocus which are useful for fabrication of three-dimensional structures.

Such systems 10 may also be particularly useful for writing waveguidesin fused silica. Fused silica is a relatively difficult glass in whichto write waveguides because of its large ionization bandgap (˜9 eV).Given a material with ionization bandgap of λ_(g), experiments indicatethat writing wavelengths include the region between about 3.0λ_(g) and5λ_(g) in order to produce waveguides with <1 dB/cm propagation loss.While it is possible to write waveguides in fused silica at longerwavelengths, operating near 3.5λ_(g) provides a wider processing window(larger separation between refractive index modification threshold anddamage threshold). Furthermore, it is possible to use up to 500 fspulses when operating near 3.5λ_(g) in comparison to <150 fs tofabricate low loss waveguides with writing wavelengths 5λ_(g).Wavelengths below 3.0λ_(g) may have reduced effectiveness in waveguidewriting and produce less desirable results, such as negative refractiveindex change. See for example, Streltsov and Borrelli in J. Opt. Soc.Am. B, vol. 19, pp. 2496-2504 (2002), which is incorporated herein byreference in its entirety. Although certain embodiments of the system 10operate in the wavelength ranges and with the pulse widths describedabove, other embodiments of the system 10 may utilize differentwavelength ranges and pulse widths (e.g., from 100 fs to 1000 fs as wellas outside this range).

Waveguide writing with 1 μm wavelength femtosecond laser sources may beaccomplished in phosphate and borosilicate glass samples. See R.Osellame et al., Opt. Lett. vol. 29, pp. 1900-1902 (2004) and M. Will etal., in SPIE Proceedings 5339, p. 168 (2004), which are alsoincorporated herein by reference in their entirety. While thefundamental wavelength of 1 μm is well suited for relatively low bandgap transparent materials such as borosilicate, soda-lime, and phosphateglasses, operating at the second harmonic frequency of embodiments ofthe laser source 200 as described above extends the utility of the laserto include a wider class of high bandgap transparent materials.

As described above, the configuration of the system may be different andvariations in the method of fabricating the waveguide are possible. FIG.4, for example, shows a system wherein the translation system comprisesa rotating or tilting mirror 30. This system 10 does not includefocusing optics. The laser beam 18 output from the laser source 12 hassufficiently reduced transverse cross-section. FIG. 5 shows a system forfabricating a waveguide that does not include a translation system. Thevisible light illuminates a mask 32 that forms a pattern on or in themedium 24 that is illuminated by the visible light. Imaging optics 34for imaging the mask 32 is also shown. The system 10 may further includeillumination optics (not shown) disposed between the laser source 12 andthe mask 32 for illuminating the mask with the laser light. In otherembodiments, a mask or reticle may be employed in addition totranslating or stepping the beam and/or the medium.

In another embodiment, a fiber waveguide may be formed by shining thevisible light through the end of a fiber. The visible light alters theindex of refraction of the fiber thereby producing a core in the fiber.The core channels visible light further along the fiber to alter thestate of the fiber in regions further along the length of the fiber. Thecore is therefore extended longitudinally along the fiber. Accordingly,the spatial region in the fiber optical medium is self-translated inthis example. Other configurations are possible.

Other types of optic elements and structures based on refractive indexmodification such as gratings, lenses, phase masks, mirrors, etc. mayalso be fabricate using apparatus and processes similar to thosedisclosed herein. Other variations are possible.

While certain embodiments of the invention have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the present invention. Accordingly, thebreadth and scope of the present invention should be defined inaccordance with the following claims and their equivalents.

1. A laser source for producing a pulsed laser beam comprising aplurality of ultrashort optical pulses having a variable repetitionrate, said laser source comprising: a fiber oscillator that outputsoptical pulses having an optical pulse width; a pulse stretcher disposedto receive said optical pulses, said pulse stretcher having dispersionthat increases said optical pulse width yielding stretched opticalpulses; a fiber amplifier disposed to receive said stretched opticalpulses, said fiber optical amplifier having gain so as to amplify saidstretched optical pulses; and an automatically adjustable gratingcompressor comprising at least one translatable bulk optical element,said compressor having dispersion that reduces the optical pulse width,said dispersion of said grating compressor being adjustable, saidgrating compressor automatically adjusting said dispersion for differentrepetition rates.
 2. The laser source of claim 1, wherein said fiberoscillator comprises doped optical fiber disposed between a pair ofreflective elements at least one of which is partially reflective. 3.The laser source of claim 1, wherein said pulse stretcher comprises anoptical fiber having dispersion.
 4. The laser source of claim 1, whereinsaid fiber amplifier comprises doped optical fiber.
 5. The laser sourceof claim 1, wherein said automatically adjustable grating compressorcomprises at least one grating and a reflective element separated by anoptical path having an optical path length that is variable so as toadjust the dispersion of said grating compressor.
 6. The laser source ofclaim 1, wherein said repetition rate is in a range of about 100 kHz toabout 5 MHz.
 7. The laser source of claim 1, wherein a pulse energy ofan amplified pulse is in a range of about 150 nJ to about 3 μJ.
 8. Alaser source for producing a pulsed laser beam comprising a plurality ofultrashort optical pulses having a variable repetition rate, said lasersource comprising: a fiber oscillator that outputs optical pulses havingan optical pulse width; and an automatically adjustable gratingcompressor comprising at least one translatable bulk optical element,said compressor having dispersion that reduces the optical pulse width,said dispersion being adjustable, said grating compressor automaticallyadjusting said dispersion for different repetition rates.
 9. The lasersource of claim 8, further comprising a pulse stretcher disposed toreceive said optical pulses, said pulse stretcher having dispersion thatincreases said optical pulse width.
 10. The laser source of claim 9,wherein said pulse stretcher comprises optical fiber having dispersion.11. The laser source of claim 8, further comprising an opticalamplifier, said optical amplifier having gain so as to amplify saidoptical pulses.
 12. The laser source of claim 11, wherein said opticalamplifier comprises doped optical fiber.
 13. The laser source of claim8, wherein said repetition rate is in a range of about 100 kHz to about5 MHz.
 14. The laser source of claim 8, wherein a pulse energy of anamplified pulse is in a range of about 150 nJ to about 3 μJ.
 15. Amethod of producing a pulsed laser beam comprising a plurality ofultrashort optical pulses having a variable repetition rate, said methodcomprising: producing optical pulses having an optical pulse width;reducing the optical pulse width thereby providing compressed opticalpulses; varying the repetition rate; and adjusting dispersion of a bulkgrating compressor automatically for said different repetition rates soas to produce the minimum pulse width.
 16. The method of claim 15,further comprising amplifying said optical pulses.
 17. The method ofclaim 16, further comprising increasing said optical pulse width priorto amplifying said stretched optical pulses.
 18. The method of claim 15,wherein said dispersion is automatically adjusted by translating agrating in the grating compressor or a reflective surface that reflectslight to a grating in the grating compressor.
 19. The method of claim15, further comprising sensing said optical pulse width.
 20. The methodof claim 19, further comprising adjusting said dispersion based on saidsensed optical pulse width.